AC-DC converter is generally known for converting an input AC voltage into an output DC voltage in order to drive a load, such as a magnetron used in a microwave or a gas discharge lamp that requires DC power to operate. FIG. 1 is a circuit block diagram showing an AC-DC converter according to the prior art. As shown in FIG. 1, an input AC voltage Vin is supplied to the input side of the AC-DC converter. A bridge rectifier 104 is connected to the input side of the AC-DC converter for rectifying the input AC voltage Vin into a full-wave rectified DC voltage. A filtering choke 106 and a filtering capacitor 112 are connected to the bridge rectifier 104 for suppressing the electromagnetic interference of the input AC voltage Vin. Besides, the AC-DC converter of FIG. 1 further includes a transformer T100 and a switch circuit 108, in which the switch circuit 108 is connected to the primary winding of the transformer T100 and consists of a high-side switch 1081 and a low-side switch 1082. The switch circuit 108 is used to control the energy transfer between the primary side and the secondary side of the transformer T100. The AC-DC converter of FIG. 1 further includes a snubber capacitor Cb1 and a resonant capacitor Cb2, in which the resonant capacitor Cb2 and the magnetizing inductance of the primary side of the transformer T100 constitute a resonant circuit. The high-side switch 1081 and the low-side switch 1082 are configured to switch in an alternate manner. That is, the high-side switch 1081 and the low-side switch 1082 are turned on and off alternately. By the alternate switching operation of the high-side switch 1081 and the low-side switch 1082, the energy stored in the primary side of the transformer T100 is transferred to the secondary side of the transformer T100 according to the switching operation of the switch circuit 108, thereby inducing an AC voltage across the secondary side of the transformer T100. The amount of the energy transferred across the transformer T100 is determined by the duty ratio of the high-side switch 1081 to the low-side switch 1082. In other words, the switching control method employed in a conventional AC-DC converter is achieved by regulating the duty cycle of the high-side switch 1081 and the duty cycle of the low-side switch 1082. Besides, the AC-DC converter of FIG. 1 further includes a secondary circuit 110 consisted of an output rectification circuit (D102, D104) and output capacitors C102 and C104. The output rectification circuit (D102, D104) is used to rectify the AC voltage induced across the secondary side of the transformer T100 into a DC voltage, and the output capacitors C102 and C104 are used to output the DC voltage to a load 120.
Also, the AC-DC converter of FIG. 1 further includes an input current sensor 130 which may be implemented by a current-sensing resistor or a current transformer. The input current sensor 130 is used to sense the input current Iin and generate an input current sensing signal V1 accordingly. The AC-DC converter of FIG. 1 further includes an input voltage sensor 140 for sensing the sinusoidal waveform of the input AC voltage Vin and outputting an input AC voltage sensing waveform signal V2 to the frequency modulation control circuit 1061. The waveform of the input AC voltage waveform sensing signal V2 is analogous to waveform of the input AC voltage Vin after being half-wave rectified or after being full-wave rectified. The AC-DC converter of FIG. 1 further includes a power control circuit 150 for comparing the input current sensing signal V1 outputted from the input current sensor 130 and an external input power control signal and outputting a control signal used to regulate the output power of the AC-DC converter. The control signal outputted from the power control circuit 150 is simply a DC signal without including the half-sinusoidal sampling signal or full-sinusoidal sampling signal of the input AC voltage Vin (the input AC voltage sensing waveform signal V2). Hence, the control signal outputted from the power control circuit 150 is used to provide information about the output power of the AC-DC converter only without the function of regulating or suppressing the harmonics of the input current Iin. The AC-DC converter of FIG. 1 further includes a switching control circuit 160 consisted of a frequency modulation control circuit 1061, an oscillator 1062, a dead-time control circuit 1063, and a switch device driver 1064. The frequency modulation control circuit 1061 is used to receive the input AC voltage waveform sensing signal V2 which is analogous to waveform of the input AC voltage Vin after being half-wave rectified or after being full-wave rectified and the control signal outputted from the power control circuit 150 for suppressing the harmonics of the input current Iin and improving the power factor of the input AC voltage Vin. The oscillator 1062 is used to generate a timing pulse signal with a sawtooth waveform according to the output of the frequency modulation control circuit 1061. The switch device driver 1064 is used to generate a square wave signal for regulating the switching frequency or the duty cycle of the switch circuit 108 according to the sawtooth-waveform timing pulse signal outputted from the oscillator 1062. The dead-time control circuit 1063 is used to control the dead time of the switch circuit 108 in order to prevent the switches 1081 and 1082 from being simultaneously turned on or off, thereby regulating the duty cycle of the ON period and the OFF period of the switch circuit 108.
Nonetheless, the conventional AC-DC converter requires an input voltage sensor to sense the variation of the input AC voltage and a dedicated frequency modulation control circuit to suppress the harmonics of the input current Iin. Also, the circuitry for suppressing the harmonics of the input current Iin and the power control circuit are separate from each other. If it is possible to simplify the circuit design of these circuitries or implemented these circuitries with low-cost elements, the cost of the AC-DC converter can be reduced and the design complexity of the AC-DC converter can be degraded. The invention is addressed to meet these needs.